The invention relates, generally, to governors and, more particularly, to an electro-hydraulic control system therefor.
The typical electronic governor acts as a feedback device on a machine or engine that is used to provide automatic control of an operating condition, for example, speed, pressure, or temperature. One common use of such governors is to automatically position the throttle of an engine to control the engine's speed.
The typical electronic governor includes of a solenoid having a plunger movable to an infinite number of positions over a pre-determined range as shown in U.S. Pat. No. 3,883,839. The plunger is connected to the machine or mechanism to be controlled such that movement of the plunger performs a controlling operation on the machine or mechanism. A coil is wound around the plunger such that when an electric current is passed through the coil the electromagnetic force will move the plunger in a first direction. A return spring is arranged in the solenoid that opposes the movement of the plunger in the first direction. By properly selecting the spring constant and by varying the current delivered to the coil, the plunger can be continuously repositioned in response to an error signal that represents the difference between the desired operating condition and actual operating condition.
The typical solenoid actuator is of a proportional variety such that the speed and distance of the plunger's movement are directly related to the magnitude of the error signal such as those shown in U.S. Pat. Nos. 3,883,839, 4,855,702, 4,321,571 and 4,164,722. In other words, when the error signal is large the plunger will move more quickly and when the error signal is small the plunger will move more slowly. In proportional solenoids, to provide a stable movement of the plunger, the spring force must intersect increasing values of current. As a result, the typical proportional governor is not very efficient because only the middle range of available force will provide useful work as will be understood by one of ordinary skill in the art.
Because the position of the plunger in the typical solenoid actuator is controlled solely by the electromagnetic force developed by the coil and the opposing force generated by the return spring, the position of the plunger can be unintentionally changed due to external forces exerted on the governor such as those caused by vibration of the machine to be controlled. In other words, the combined forces of the spring and coil do not lock the plunger in a stationary position. Rather, the typical electronic governor has some "softness" such that undesired variations in the plunger's position can result. This "softness" adversely affects the precision with which the plunger can be positioned. Moreover, conventional proportional electronic governors require an integrator build u to generate high output power even for small errors. This build up requires time and often results in overshoot or instability of the plunger.
Finally, in order for the prior art solenoids to maintain the plunger's desired position near high output conditions, relatively high current must be constantly provided to the coil to balance the force generated by the compression spring. As a result, the prior art devices continuously draw relatively high current, even when the solenoid is at an equilibrium position.
The typical hydraulic governor avoids these undesirable and unintentional changes in the plunger's position by using hydraulic fluid to move the plunger and hold it in the desired position. While these hydraulic systems avoid the "softness" of the electronic systems, they utilize complex and expensive hydraulic pumps and mechanical stabilizing mechanisms including various arrangements of pistons, linkages, springs, reservoirs and filters. As a result, these hydraulic systems are expensive to manufacture and maintain and are difficult to install.
Thus, an electro-hydraulic control for a governor that eliminates the need for proportional output solenoids, hydraulic pumping systems, mechanical stabilizing mechanisms, and high power drain is desired.